The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed:
For example, an address which is constant is offsettable; so is an address that is the sum of a register and a constant (as long as a slightly larger constant is also within the range of address-offsets supported by the machine); but an autoincrement or autodecrement address is not offsettable. More complicated indirect/indexed addresses may or may not be offsettable depending on the other addressing modes that the machine supports.
Note that in an output operand which can be matched by another
operand, the constraint letter
o is valid only when accompanied
< (if the target machine has predecrement addressing)
> (if the target machine has preincrement addressing).
mconstraint but not the
Pmay be defined in a machine-dependent fashion to permit immediate integer operands with explicit integer values in specified ranges. For example, on the 68000,
Iis defined to stand for the range of values 1 to 8. This is the range permitted as a shift count in the shift instructions.
const_double) is allowed, but only if the target floating point format is the same as that of the host machine (on which the compiler is running).
const_double) is allowed.
Hmay be defined in a machine-dependent fashion to permit immediate floating operands in particular ranges of values.
This might appear strange; if an insn allows a constant operand with a
value not known at compile time, it certainly must allow any known
value. So why use
s instead of
i? Sometimes it allows
better code to be generated.
For example, on the 68000 in a fullword instruction it is possible to
use an immediate operand; but if the immediate value is between -128
and 127, better code results from loading the value into a register and
using the register. This is because the load into the register can be
done with a
moveq instruction. We arrange for this to happen
by defining the letter
K to mean "any integer outside the
range -128 to 127", and then specifying
Ks in the operand
This number is allowed to be more than a single digit. If multiple
digits are encountered consecutavely, they are interpreted as a single
decimal integer. There is scant chance for ambiguity, since to-date
it has never been desirable that
10 be interpreted as matching
either operand 1 or operand 0. Should this be desired, one
can use multiple alternatives instead.
This is called a matching constraint and what it really means is
that the assembler has only a single operand that fills two roles
asm distinguishes. For example, an add instruction uses
two input operands and an output operand, but on most CISC
machines an add instruction really has only two operands, one of them an
Matching constraints are used in these circumstances.
More precisely, the two operands that match must include one input-only
operand and one output-only operand. Moreover, the digit must be a
smaller number than the number of the operand that uses it in the
p in the constraint must be accompanied by
as the predicate in the
match_operand. This predicate interprets
the mode specified in the
match_operand as the mode of the memory
reference for which the address would be valid.
fare defined on the 68000/68020 to stand for data, address and floating point registers.